The present invention relates to a laser beam emitting apparatus employing a semiconductor laser or the like and, more particularly, to a laser beam emitting apparatus capable of operating at low power consumption.
Laser beam emitting apparatuss employing a semiconductor laser have been used in various fields. Recent rapid progress of laser technology made possible the outdoor use of battery-powered laser beam emitting apparatuss as surveying instruments as well as laser beam emitting apparatuss using commercial power.
An example of a conventional laser beam emitting apparatus 10000 will be described with reference to FIG. 27. The laser beam emitting apparatus 10000 comprises a laser head 1000, a laser diode driving unit (LD driving unit) 2000, a Peltier device driving unit 3000, a nonlinear optical medium temperature detecting unit (KTP temperature detecting unit) 4000, and an analog controller 5000. The laser head 1000 includes a pumping laser diode 1100, a laser crystal (YVO.sub.4) 1200, a nonlinear optical medium 1300, an output mirror (OC mirror) 1400, and a Peltier device 1500. The pumping laser diode 1100 is used for producing a laser beam and serves as a pumping source for generating a fundamental wave. The laser crystal 1200 is a medium having a negative temperature and amplifies light. The laser crystal 1200 employs Nd:YVO.sub.4 having an oscillation line at 1064 nm. The laser crystal 1200 may be YAG (yttrium/aluminum/garnet) doped with Nd.sup.3+ ions instead of Nd:YVO.sub.4. YAG has oscillation lines at 946 nm, 1064 nm and 1319 nm. Ti:sapphire having an oscillation line at a wavelength in the range of 700 to 900 nm may be used.
A first dielectric reflecting film is formed for the pumping laser diode 1100 for pumping the laser crystal 1200. The first dielectric reflecting film has a high transmittance to laser radiation emitted by the pumping laser diode 1100, has a high reflectivity to the oscillation wavelength of the laser crystal 1200 and the second harmonic. The output mirror 1400 is disposed opposite to the laser crystal 1200 on which the first dielectric reflecting film is formed. The surface of the output mirror 1400 on the side of the laser crystal 1200 is formed in a concave spherical surface of an appropriate radius and a second dielectric reflecting film is formed on the concave spherical surface. The second dielectric reflecting film has a high reflectivity to the oscillation wavelength of the laser crystal 1200, and has a high transmittance to the second harmonic.
When the first dielectric reflecting film of the laser crystal 1200 and the output mirror 1400 are combined and the laser crystal 1200 is pumped by the light flux emitted by the pumping laser diode 1100, light reciprocates between the first dielectric reflecting film of the laser crystal 1200 and the output mirror 1400. Thus, light can be confined for a long time to amplify the light by resonance. In this conventional laser beam emitting apparatus 10000, the nonlinear optical medium 1300 is inserted in an optical resonator consisting of the first dielectric reflecting film of the laser crystal 1200, and the output mirror 1400.
Nonlinear optical effect will briefly be described. A body is polarized when an electric field is applied thereto. The intensity of polarization is proportional to the electric field if the intensity of the electric field is low. In the case of intense coherent light, such as a laser beam, the proportional relation between electric field and polarization is broken, and polarization components proportional to the square and the cube of the intensity of the electric field become dominant.
Therefore, in the nonlinear optical medium 1300, polarization caused by a light wave includes a polarization component proportional to the square and the cube of the intensity of a photoelectric field. Light waves of different frequencies are coupled by this nonlinear polarization, and the second harmonic of a frequency twice the frequency of light is generated. The generation of the second harmonic is called second harmonic generation (SHG).
Since the nonlinear optical medium 1300 is inserted in the optical resonator consisting of the laser crystal 1200 and the output mirror 1400, the higher harmonic generation in this laser beam emitting apparatus 10000 is called internal second harmonic generation (internal SHG). Since the conversion output is proportional to the square of the power of the fundamental wave, the light of a high intensity produced in the optical resonator can directly be used.
The nonlinear optical medium 1300 is, for example, KTP (KTiOPO.sub.4, titanium potassium phosphate), BBO (.beta.-BaB.sub.2 O.sub.4, .beta. lithium borate) or LBO (LiB.sub.3 O.sub.5, lithium triborate). Mainly, a wave of 1064 nm in wavelength is converted into a wave of 532 nm in wavelength. When the nonlinear optical medium 1300 is KNbO.sub.3 (potassium niobate), a wave of 946 nm in wavelength is converted into a wave of 473 nm in wavelength.
In this laser beam emitting apparatus 10000, the laser diode driving unit (LD driving unit) 2000 drives the pumping laser diode 1100 by a constant DC current. The analog controller 5000 controls the Peltier device driving unit 3000 on the basis of a detection signal provided by the KTP temperature detecting unit 4000 so that the temperature of the nonlinear medium 1300 is equal to a predetermined temperature determined when the laser resonator was adjusted. The Peltier device driving unit 3000 drives the Peltier device 1500 to maintain the nonlinear optical medium 1300 at the predetermined temperature determined when the laser resonator was adjusted.
The analog controller 5000 carries out a simple analog control method.
As shown in FIG. 28, the laser diode driving unit 2000 comprises a transistor 2100, and a resistor 2200 connected to the base (B) of the transistor 2100. The transistor 2100 serves as a current amplifier. The collector current of the transistor 2100 is controlled by controlling an input voltage applied to the transistor 2100 by the analog controller 5000 to control the current supplied to the pumping laser diode 1100 from a power supply 6000.
As shown in FIG. 29, the Peltier device driving unit 3000 comprises an npn transistor 3100, a pnp transistor 3200 and a resistor 3300. The Peltier device driving unit 3000 is able to control the direction and the intensity of a current to be supplied to the Peltier device 1500 by using two power supplies 6100 and 6200, the npn transistor 3100 and the pnp transistor 3200.
This conventional laser beam emitting apparatus 10000 is capable of generating laser light of a wavelength different from that of the pumping laser light. However, the laser beam emitting apparatus 10000 consumes much power and hence, when incorporated into an instrument and powered by a battery, is able to operate only a relatively short time before the battery is exhausted. Accordingly, there has been an earnest desire for the development of a laser beam emitting apparatus capable of efficiently generating laser light at low power consumption and of continuously operating for a remarkably extended period of time.
In the conventional laser beam emitting apparatus 10000, the temperature of the laser is regulated at a temperature determined when the laser resonator was adjusted, and the Peltier device driving unit 3000 drives the Peltier device 1500 to adjust the temperature of the nonlinear optical medium to the temperature determined when the laser resonator was adjusted if the ambient temperature changes from that when the laser resonator was adjusted. The current flowing through the Peltier device 1500 increases as the difference between the ambient temperature and the temperature of the nonlinear optical medium when the laser resonator was adjusted becomes large. Therefore, the laser beam emitting apparatus 10000 has an inevitable disadvantage that it is difficult to use the laser beam emitting apparatus 10000 in a battery power supply mode in an environment requiring temperature control in a wide temperature range, such as an outdoor environment. In particular, it has been practically impossible to employ the conventional laser beam emitting apparatus 10000 in a portable instrument, such as a surveying instrument or a portable laser instrument.
The resonant state of the laser resonator changes with time and, in some cases, green laser light is generated efficiently at a temperature different from that determined during initial adjustment. The conventional laser beam emitting apparatus 10000 is unable to deal with such a condition and is unable to generate green laser light at a high efficiency.
Since the transistor 2100 of the laser driving unit (LD driving unit) 2000 operates in an unsaturated state while only a low operating current is necessary, the transistor 2100 wastes power P=V.sub.CE .times.I.sub.C, where V.sub.CE is voltage across the collector (C) and the emitter (E) of the transistor 2100 and I.sub.C is collector current, and generates heat.
The Peltier device driving unit 3000, similarly to the laser diode driving unit 2000, operates the transistor 3100 or the transistor 3200 in an unsaturated state. Therefore, the transistor 3100 or the transistor 3200 wastes power P=V.sub.CE .times.I.sub.C, where V.sub.CE is voltage across the collector (C) and the emitter (E) of the transistor 3100 or the transistor 3200, and I.sub.C is collector current, and generates heat.
Furthermore, the Peltier device driving unit 3000 needs the two power supplies 6100 and 6200 respectively for cooling and heating, which increases the number of parts and the costs, and makes the miniaturization of the laser beam emitting apparatus 10000 difficult.